All cells rely on the regulated movement of inorganic ions across cell membranes to perform essential physiological functions. Electrical excitability, synaptic plasticity, and signal transduction are examples of processes in which changes in ion concentration play a critical role. In general, the ion channels that permit these changes are proteinaceious pores consisting of one or multiple subunits, each containing two or more membrane-spanning domains. Most ion channels have selectivity for specific ions, primarily Na+, K+, Ca2+, or Cl−, by virtue of physical preferences for size and charge. Electrochemical forces, rather than active transport, drive ions across membranes, thus a single channel may allow the passage of millions of ions per second. Channel opening, or “gating” is tightly controlled by changes in voltage or by ligand binding, depending on the subclass of channel. Ion channels are attractive therapeutic targets due to their involvement in so many physiological processes, yet the generation of drugs with specificity for particular channels in particular tissue types remains a major challenge.
Voltage-gated ion channels open in response to changes in membrane potential. For example, depolarization of excitable cells such as neurons result in a transient influx of Na+ ions, which propagates nerve impulses. This change in Na+ concentration is sensed by voltage-gated K+ channels, which then allow an efflux of K+ ions. The efflux of K+ ions repolarizes the membrane. Other cell types rely on voltage-gated Ca2+ channels to generate action potentials. Voltage-gated ion channels also perform important functions in non-excitable cells, such as the regulation of secretory, homeostatic, and mitogenic processes. Ligand-gated ion channels can be opened by extracellular stimuli such as neurotransmitters (e.g., glutamate, serotonin, acetylcholine), or intracellular stimuli (e.g. cAMP, Ca2+, and phosphorylation).
The Cav2 family of voltage-gated calcium channels consists of 3 main subtypes Cav2.1 (P or Q-type calcium currents), Cav2.2 (N-type calcium currents) and Cav2.3 (R-type calcium currents). These currents are found almost exclusively in the central nerves system (CNS), peripheral nerves system (PNS) and neuroendocrine cells and constitute the predominant forms of presynaptic voltage-gated calcium current. Presynaptic calcium entry is modulated by many types of G-protein coupled receptors (GPCRs) and modulation of Cav2 channels is a widespread and highly efficacious means of regulating neurotransmission. The subunit composition of the Cav2 channels is defined by their α1 subunit, which forms the pore and contains the voltage-sensing gates (α12.1, α12.2 and α12.3, also known as α1A, α1B and α1E respectively) and the β, α2δ and γ subunits.
Genetic or pharmacological perturbations in ion channel function can have dramatic clinical consequences. Long QT syndrome, epilepsy, cystic fibrosis, and episodic ataxia are a few examples of heritable diseases resulting from mutations in ion channel subunits. Toxic side affects such as arrhythmia and seizure which are triggered by certain drugs are due to interference with ion channel function (Sirois, J. E. and, Atchison, W. D., Neurotoxicology 1996; 17(1):63-84; Keating, M. T., Science 1996 272:681-685). Drugs are useful for the therapeutic modulation of ion channel activity, and have applications in treatment of many pathological conditions, including hypertension, angina pectoris, myocardial ischemia, asthma, bladder overactivity, alopecia, pain, heart failure, dysmenorrhea, type II diabetes, arrhythmia, graft rejection, seizure, convulsions, epilepsy, stroke, gastric hypermotility, psychoses, cancer, muscular dystrophy, and narcolepsy (Coghlan, M. J., et al. J. Med Chem. 2001, 44:1627-1653; Ackerman. M. J., and Clapham, D. E. N. Eng. J. Med. 1997, 336:1575-1586). The growing number of identified ion channels and understanding of their complexity will assist in future efforts at therapies, which modify ion channel function.
Therapeutic modulation of Cav2 channel activity has applications in treatment of many pathological conditions. All primary sensory afferents provide input to neurons in the dorsal horns of the spinal cord and in dorsal root ganglia neurons in the dorsal horn and calcium influx through Cav2.2 channels triggers the release of neurotransmitters form presynaptic nerve terminals in the spinal cord. Hence blockade of Cav2.2 channels is expected to be broadly efficacious because these channels are in a common pathway downstream form the wide variety of receptors that mediate pain (Julius, D. and Basbaum, A. I. Nature 2001, 413:203-216). Indeed, intrathecal injection of Cav2.2 selective conopeptide ziconitide (SNX-111) has been shown to be broadly effective against both neuropathic pain and inflammatory pain in animals and man (Bowersox, S. S. et al, J Pharmacol Exp Ther 1996, 279:1243-1249). Ziconotide has also been shown to be highly effective as a neuroprotective agent in rat models of global or focal ischemia (Colburne, F. et al, Stroke 1999, 30:662-668). Thus it is reasonable to conclude that modulation of Cav2.2 has implications in the treatment of neuroprotection/stroke.
Cav2.2 channels are found in the periphery and mediate catecholamine release from sympathetic neurons and adrenal chroffin cells. Some forms of hypertension result from elevated sympathetic tone and Cav2.2 modulators could be particularly effective in treating this disorder. Although complete block of Cav2.2 can cause hypotension or impair baroreceptor reflexes, partial inhibition by Cav2.2 modulators might reduce hypertension with minimal reflex tachycardia (Uneyama, O. D. Int. J. Mol. Med. 1999 3:455-466).
Overactive bladder (OAB) is characterized by storage symptoms such as urgency, frequency and nocturia, with or without urge incontinence, resulting from the overactivity of the detrusor muscle in the bladder. OAB can lead to urge incontinence. The etiology of OAB and painful bladder syndrome is unknown, although disturbances in nerves, smooth muscle and urothelium can cause OAB (Steers, W. Rev Urol, 4:S7-S18). There is evidence to suggest that reduction of bladder hyperactivity may be indirectly effected by inhibition of Cav2.2 and/or Cav1 channels.
The localization of Cav2.1 channels in the superficial laminae of the dorsal horn of the spinal cord suggests involvement of these channels in the perception and maintenance of certain forms of pain (Vanegas, H. and Schaible, H. Pain 2000, 85:9-18. Complete elimination of Cav2.1 calcium currents alters synaptic transmission, resulting in severe ataxia. Gabapentin has been used clinically for many years as an add-on therapy for the treatment of epilepsy. In recent years, it has emerged as a leading treatment of neuropathic pain. Clinical trials have shown gabapentin to be effective for the treatment of post-herpetic neuralgia, diabetic neuropathy, trigeminal neuralgia, migrane and fibromyalgia (Mellegers, P. G. et al Clin J Pain 2001, 17:284-295). Gabapentin was designed as a metabolically stable GABA mimetic, but most studies find no effect on the GABA receptors. The α2δ subunit of the Cav2.1 channel has been identified as a high affinity binding site for gabapentin in the CNS. There is evidence that suggests that gabapentin could inhibit neurotransmission in the spinal cord by interfering with the function of the α2δ subunits thereby inhibiting presynaptic calcium currents.